Crystal oscillators, in particular Pierce oscillators, are widely used in the art to provide stable clock signals. There is a general need to design circuitry that is both suitable for all kinds of applications and highly efficient with respect to the specific requirements of each application. Specifically, electronic devices or electronic circuitry which provide a crystal oscillator should be made to support as many crystals as possible while still providing a constant frequency, high (in absolute value) negative impedance, low phase noise, low power consumption and low power dissipation on the crystal. For integrated solutions, the integrated components and circuitry should be immune to all kinds of noise sources. A large frequency pulling range is also desirable.
A detailed discussion and a prior art solution for a Pierce oscillator is given in Vittoz, et al., “High-Performance Crystal Oscillator Circuits: Theory and Application,” IEEE Journal of Solid-State Circuits, Vol. 23, No. 3, June 1988, p. 774. The crystal oscillator circuit presented by Vittoz includes an amplifier stage, an amplitude control stage, an output voltage regulator and an output amplifier. The amplitude control is necessary in order to provide a suitable negative impedance, when looking from the crystal into the respective input pins of the crystal oscillator circuit. The negative impedance should be equal to the equivalent serial resistance (ESR) of the crystal. A negative impedance that is too large has the disadvantages of high power dissipation and fast degradation of the crystal. If the negative impedance is smaller than the ESR, the oscillation may die out or there may be no oscillation at all.
The mechanism disclosed by Vittoz to control the ratio between the negative impedance and the ESR is based on control of the bias current through a transistor used as an amplifying transistor for the crystal. The bias current is controlled by an amplitude control circuit (ACC). The ACC monitors the amplitude of the oscillation and generates a control voltage to reduce or increase the amplitude. The control voltage is used to modify the current through the amplifying transistor. Due to the specific control mechanism used by Vittoz, however, the conversion gain is very high. Accordingly, the noise generated by the ACC is amplified and fed into the crystal oscillation loop. This severely degrades the phase noise performance. Furthermore, the crystal oscillator circuit suggested by Vittoz has only limited immunity to substrate noise or cross-talking when implemented on an integrated circuit, in particular in a system on chip (SoC). Finally, the crystal oscillator suggested by Vittoz has only a small pulling range and poor adaptability to multiple different crystals.